There are many more powerful PDF and image editors, yet plenty of us use Preview for those basic little tasks, where we don’t need the complications of a heavyweight. Preview isn’t without its faults, though. It can mutilate PDF annotations made by other apps, and sometimes unexpected things happen. To deal with those it has one feature that’s almost unique among PDF and image editors: it automatically saves versions using the macOS versioning system. This article explains how to make use of that.

Ask to save changes when closing documents

One important control over the behaviour of Preview and many other apps that you may not be aware of is this in the Windows section of Desktop & Dock settings. When combined with apps that use the macOS versioning system its effects are significant.

Ignoring versioning for the moment, when this setting is turned on, if you go to close an open document that has unsaved changes, before it’s closed you’ll be asked whether you want to save that changed document. This is a long-standing safety net that continues to protect us from losing lots of work by accident.

When this setting is turned off, apps may still ask you whether you want to save unsaved changes before closing a document unless the app uses the macOS versioning system. In that case, the app automatically saves a new version without offering any option. This might appear inappropriate, but as that’s non-destructive, it avoids interrupting your workflow: you can always revert to the previous version of that document, provided that you’re aware that it has been saved automatically.

If you’re not aware of what’s going on here, and how the versioning system works, this can cause odd effects you can demonstrate using Preview.

Preview’s saving grace

To see how Preview handles versions and the effect of that setting, find a copy of a suitable PDF or image file and duplicate it in the Finder. Name one copy something like testSavesOff, and the other testSavesOn. Set Ask to save changes when closing documents off to begin with, and open testSavesOff in Preview.

Now perform some destructive editing on that document without saving it, here a radical crop.

Leave it a couple of minutes before closing the document. Following that setting, Preview shouldn’t ask you whether to save the changed document, but will simply close it. Then quit Preview.

Now set Ask to save changes when closing documents on, and repeat the same sequence with testSavesOn instead. When you try to close that document, Preview should now ask you whether you want to save the changed document, to which you should click on Revert Changes to set the document back to its previous state instead.

Inspect the two documents using Quick Look and you’ll see that testSavesOn hasn’t changed, but testSavesOff has, although in neither case did you save those changes yourself.

All in the versions

Although you can see what has happened using Preview’s Revert To menu command, the clearest way to see what has happened to those two documents is to open them using my free Revisionist, which shows their saved versions.

testSavesOn has three saved versions. The current one is the same as the original, but the second version shows the destructive edit that you didn’t save.

testSavesOff has two saved versions, the original, and the current version is that after the destructive edit, which you also didn’t save.

Thanks to Preview’s use of the macOS versioning system, either way you’ve still got access to both versions of that document.

One point to note, though, is that the versioning system doesn’t automatically clean up old versions for you. When you’ve finished editing a document in Preview and don’t want to retain its old versions, delete them either using Revert To in Preview, or with Revisionist. Versions are only retained for the original document as long as it’s stored on the same volume. So you can also wipe old versions by duplicating a document and trashing the original, or copying it to another volume. Once they’re gone, you can’t restore them, as not even Time Machine can back up versions.

In this last batch of updates to my apps for the next few weeks, there are four more popular tools, covering Unicode normalisation, appexes, logs, and document versions.

Unicode normalisation

Perhaps the earliest problem with APFS was its lack of Unicode normalisation for file and folder names. This has been a standard way to address accented and other characters that appear identical but have different codes. Apple addressed that, first in providing a normalisation layer on top, then by incorporating it into APFS. However, it can still prove a problem, both within apps and when working with other file systems. Apfelstrudel is a simple app that reveals any potential problems with normalisation, and helps you use the form most appropriate. Version 1.6 has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards.

Apfelstrudel 1.6 is now available from here: apfelstrudel16
from its Product Page, and via its auto-update mechanism.

Appexes

App extensions, or appexes, are numerous in recent versions of macOS, and widely used by apps. This simple utility shows all those managed by PlugInKit, complete with their UUIDs, to help you manage them. Version 1.1 has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Sonoma 14.6 onwards.

AppexIndexer 1.1 is now available from here: appexindexer11
and from its Product Page. It doesn’t yet support auto-update.

Logs

Until I started development of LogUI, Ulbow was my preferred app for browsing the Unified log. It has extensive features, with full support for the use of predicates, a chart showing the most frequent sources of log entries, and support for creating and using logarchives, including those from iOS and iPadOS. Unlike LogUI, it uses the log command to obtain log extracts, enabling it to show entry times in nanoseconds. It also displays extracts in Rich Text rather than as a list. Version 1.11 fixes a crashing bug when handling some logarchives, has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards, and is recommended for all users.

Ulbow 1.11 is now available from here: ulbow111
from its Product Page, and via its auto-update mechanism.

Document versions

While Revisionist (also recently updated) provides a suite of tools to work with macOS document versions, Versatility handles one of those tasks with greater ease, creating version archives, and reconstituting them into documents. Simply drop a file onto its window and it will be converted into a folder containing each saved version as a separate document. Drop one of those archive folders onto its window and it will be reconstituted into a document with all those previous versions. This makes it simple to preserve versions when moving documents between volumes or computers, and for archival purposes. Version 1.1 has been rebuilt with a new app icon ready for macOS 26 Tahoe, and supports macOS from Big Sur onwards.

Versatility 1.1 is now available from here: versatility11
from its Product Page, and via its auto-update mechanism.

Next updates

Most of my other apps that haven’t yet been updated for Tahoe should still run perfectly well, although their app icons won’t appear the same as before. I’m now turning my attention to the successor to SilentKnight and Skint, and my virtualisers Viable, ViableS, Vimy and Liviable. Once I’m done with those, I’ll return and complete my other apps.

Enjoy!

This next batch of updates to my apps includes more popular tools, covering iCloud, document versions, sparse bundles, and Time Machine backups.

iCloud

Cirrus gives you detailed insight into what’s stored in iCloud Drive, provides a ready-made log browser for checking what’s going on, and a simple test for syncing. Version 1.16 has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards.

Cirrus 1.16 is now available from here: cirrus116
from its Product Page, and via its auto-update mechanism.

Document versions

Revisionist gives you direct access to versions of documents saved automatically by macOS, and a powerful suite of tools to work with them. You can run checks to discover which documents have saved versions, then browse those, previewing them with Quick Look. It can save individual versions as new files, and create archive folders containing all versions, that can be reconstituted into the original with those versions preserved. Version 1.10 has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards.

Revisionist 1.10 is now available from here: revisionist110
from its Product Page, and via its auto-update mechanism.

Sparse bundle disk images

Spundle creates and maintains sparse bundle disk images, offering a range of supported file systems, and features such as compaction to maintain their efficiency. Version 1.9 has an overhauled window, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards.

Spundle 1.9 is now available from here: spundle19
from its Product Page, and via its auto-update mechanism.

Time Machine backups

The Time Machine Mechanic, T2M2, is the standard utility for checking your Mac’s Time Machine backups. It checks and reports on their performance, free space on backup storage, how much has been transferred in each backup, and much more. Version 2.03 has an overhauled interface, and has been rebuilt with a new app icon ready for macOS 26 Tahoe. This version supports macOS from Big Sur onwards, backing up to APFS.

Depending on any changes finalised in the full public release of Tahoe later this year, I may need to make further adjustments to its code.

T2M2 2.03 is now available from here: t2m2203
from its Product Page, and via its auto-update mechanism.

Enjoy!

Although there was no ambiguity in Apple’s announcement that later this year it will be releasing macOS 26 Tahoe, together with version 26 of its other operating systems, there have been claims that this might just be a ‘marketing version’ and not really the case. There is some evidence that could be misinterpreted as confirming that, where some of Apple’s developer web pages refer to macOS 16.

But others choose to differ.

Cast your mind back five years to macOS 11 Big Sur, when what had been expected to be macOS 10.16 but was announced as 11.0 instead. That had the potential to upset a lot of code and scripts that had become used to checking the minor but not major version number. Apple foresaw those problems, and devised an ingenious scheme that allowed Big Sur to be simultaneously both 10.16 and 11.0. It’s hardly surprising that has been implemented once again for Tahoe.

Rules

There are two fundamental rules provided by Apple:

• In compiled languages, the version returned by macOS depends on the SDK which the software has been built against. When built against the 15 SDK or earlier, Tahoe returns 16 for compatibility with previous numbering and all existing apps; when built against the 26.0 SDK, it returns 26.0 for forward compatibility.
• In scripted languages run within a shell environment, there’s an environmental variable to control the version number given. Set SYSTEM_VERSION_COMPAT=1 and Tahoe returns 16; leave that variable unset, or SYSTEM_VERSION_COMPAT=0, and it returns 26.

AppleScript

Move a script across to Tahoe, and it will be compiled in the 26 environment, so
system version of (system info)
returns 26.0, as will that code inside an AppleScript app built on Tahoe.

Scripts and other languages

One method commonly used to look up the macOS version number is to obtain the string value for the ProductVersion key in /System/Library/CoreServices/SystemVersion.plist. However, depending on the environment of the caller, Tahoe plays tricks with that file, which should return a version of 26.0. If the caller has set SYSTEM_VERSION_COMPAT=1, then the version number returned isn’t obtained from that property list at all, but its companion SystemVersionCompat.plist, which is 16.0.

You can test this at the command line, by entering the two commands
SYSTEM_VERSION_COMPAT=1 cat /System/Library/CoreServices/SystemVersion.plist
and
SYSTEM_VERSION_COMPAT=0 cat /System/Library/CoreServices/SystemVersion.plist

Which is it – 16 or 26?

macOS Tahoe is very definitely, and not just for marketing purposes, macOS 26, but depending on how you ask that question, it could pretend to be 16 if you wish.